Radio Frequency (RF) filters are widely used in modern communication systems to perform filtering of signals. In a Frequency Division Duplex (FDD) working system, filters also function to combine the power of a number of channels into a composite wideband signal for transmission via a common antenna.
The traditional technology for RF filter design is based on coaxial resonators. They have been developed for decades and have been operating on most of communication systems, especially on Radio Base Stations (RBS). They are easy to design, manufacture and maintain, but is not a unique solution. One type of RF filters is ceramic filters or Dielectric Resonator (DR) filters. Using DR filters enables manufacturers to shrink the RF filters substantially. By shrinking the filter volume, smaller products may be designed and a DR filter is assembled by arranging ceramic parts, such as pucks, discs, rods or the like, into a rigid housing or a sheet metal housing.
The DR filters are becoming more and more popular for wireless communication system because of the better performance. It has been proven that DR filters have many advantages compared with traditional coaxial filters. One of the most attractive properties lies in the fact that it can make the RF filter compact in size.
For Transverse Magnetic (TM) mode resonators, DR filters can be made even smaller if step resonators are applied, so called Step Diameter Section (SDS) structure. But this advantage comes with one obvious drawback, from the technical point of view, that of poor spurious performance. The spurious band of DR filters is much closer to the operating frequency band than that of traditional filters. The spurious problem is attributed to the working mode of a dielectric resonator. Theoretically, one dielectric resonator can support a variety of resonant modes, which causes the frequency spacing between the desired fundamental resonant mode frequency and higher-order resonant modes frequency to be quite narrow.
In order to benefit from reducing filter volume, dielectric resonators are more and more widely used in filter design, and a Low Pass Filter (LPF) is always applied in a DR filter to suppress spurious emissions. But due to the LPF's long transition band from pass band to stop band, the frequency of higher-order resonant modes for a resonator; (that is, the spurious band for a filter accordingly, close to the frequency of fundamental resonant mode) is difficult to suppress, and is still very harmful to filter unit (FU) design.